1. Technical Field
The present invention relates to a scanned-slot X-ray imaging system having a first collimator and a second collimator arranged at a first distance and a second distance, respectively, from a radiation source. Each collimator is provided with a slot and a detector located under the second collimator slot, with the slot of the second collimator being wider than the slot of the first collimator, and the detector under the second slot being wider than the first collimator slot and the second collimator slot.
2. Background Information
Typical X-ray imaging systems consist of an X-ray source in front of an object and an area detector behind the object for registering the image. However, this set-up is sensitive to background noise in the form of Compton scattered radiation. Existing methods to remove this background noise are inefficient, and also tend to remove part of the primary X-rays containing the image information. This result in requiring dose increases exceeding a factor of two (2) or more.
One solution to this problem is a scanned-slot set up. With this solution, a pre-collimator slot before the object shapes the X-ray beam so that it matches the active detector area. The slot is mechanically moved in order to image the entire object. It is also possible to move or have the object move with respect to the slot. However, this can be more inconvenient as the object is usually heavier than the mechanics for the slot. Still, the solution is advantageous since only a narrow fan-beam crosses the object at any single time and the area of the secondary collimator is small relative to the area of the captured image, thereby minimizing the amount of Compton scattered X-rays. Another advantage with the scanned-slot approach is that the required detector area is much smaller. This reduces costs, and also enables the use of more expensive and efficient detector materials if desired.
A drawback with the scanned-slot geometry is that only a small fraction of the X-rays from the source is actually used to form the image. As a result, the time for image acquisition is extended, requiring the X-ray tube to be turned on for a longer period of time. A way of mitigating this problem and achieving a practical system is to use a multi-slot collimator with different detector arrays under each slot. However, this makes image acquisition difficult since information from the different detectors has to be combined together into one image without any visible artifacts, such as border lines between areas where different detectors are used.
One of the most important constraints of medical X-ray imaging systems is avoiding patient exposure to X-rays in areas where there is no active detector for registering the X-rays. Such exposure only leads to an unnecessary dose increase. In a multi-slot setup, alignment is crucial since the detectors need to cover the full area under each slot.
International Patent Application No. WO 82/01124 describes an apparatus having a planar, proximity type X-ray image intensifier for detecting a fan beam of X-rays and for producing an intensified output visible light image on an output display screen. This fan beam is sensed by a scannable, linear array of solid state diode detectors. In one embodiment, a pair of side-by-side arrays are utilized in eliminating flare effects in the display screen. One of the linear arrays looks at the line signal on the output screen, while the second linear array looks at a location on the output screen adjacent and parallel to the line signal. A net signal is derived by subtracting the signals from adjacent elements of the two parallel arrays so that signal flare in the image intensifier tube is removed. In another embodiment, display screen flare is eliminated by covering the vacuum side of the display screen with metal having a thickness sufficient to dissipate one third of the kinetic energy of photo-electrons passing through it.
U.S. Pat. No. 4,649,559 discloses a large area, digital radiography apparatus. In this patent, a pre-scatter and a post-scatter collimator are moved simultaneously with an X-ray image intensifier tube, whose output display is scanned by a stationary scanning camera producing a digitized X-ray image over a large cross-sectional area of the patient.
It is important that the detectors cover the whole X-ray-imaging object in the direction orthogonal to the scan and without any gaps between detectors. For semiconductor detectors, this is an engineering challenge since there is always a dead area close to the edge of the detector. This is caused by mechanical damage when cutting the detectors on the wafer. Usually, a guard-ring is placed between the edge and the active detector area in order to sink leak current emanating from the mechanical damages. Ideally, none of this dead area is exposed to the diagnostic X-rays.
One object of the present invention is to provide a setup for multi-slot medical X-ray imaging that greatly simplifies the alignment, while also presenting a method for tiling different semiconductor detectors to cover the whole slot without introducing any dead area in between detectors.
Another object of the present invention is to allow for a misalignment with respect to the central symmetry line with less than a safety factor so that no primary radiation is lost in the post collimator.
These objects are accomplished by arranging the previously mentioned slot of the second collimator with a width not less than a pre-determined safety margin. The product of the slot width of the first collimator and the second distance is divided by the first distance for allowing a misalignment with respect to a central symmetry line of the slots.
Furthermore, the system can comprise a plurality of first and second collimators and detectors arranged side-by-side, thereby enabling a multi-slot scan.
In a preferred embodiment, the detector is a semiconductor detector and can be oriented so that an edge faces the incident X-rays. However, the detector can also be a film-screen combination, a CCD coupled to a scintillator through optical fiber bundles, or a gas detector.
If the detector is a gas detector, it can have a drift field for electrons released through interactions with the X-rays to drift to the edge of the detector where the signal is amplified and registered.
The invention also includes, in a scanned-slot X-ray imaging system, a first collimator and a second collimator arranged in a first distance and a second distance, respectively, from a radiation source. Each collimator is provided with a slot and a detector located under the second collimator slot, with the second collimator slot being wider than the first collimator slot, and the detector under the second slot being wider than the first collimator slot and the second collimator slot. The invention further provides for a method for allowing a misalignment with respect to a central symmetry line of the slots. The method comprises arranging the second collimator slot so that its width is not less than a safety margin. The product of the first collimator slot width and the second distance is divided by the first distance. Moreover, the collimators can be arranged so that a dead area on the detector is not exposed to the X-ray.